There is no consensus about this because there is no one fabric with consistent performance called "silnylon."
There are a number of manufacturers, and many of the sources are in Asia and unknown to consumers. Even for known sources, the quality varies from batch to batch, and the quality of the coatings is not consistent. As a do-it-yourselfer, I've found the only solution is to order a small quantity from a usually reliable supplier, test it, and if it is good, immediately place a larger order hoping to get the same batch.
Tent manufacturers have more resources, but are still subject to these variations. Added to that, the function of the fabric over time remains unclear. Coatings will "cure," and it is unclear what products have the best useful life.
Wrestling with these headaches, I've decided that for tents, dome or multiple hoop shapes with internal line tighteners are the only way to go with silicon coated nylon, even if it is the best quality I can find, and even if the tent has a full inner. This is consistent with the posters who have found less sagging issues with the hoop designs.
There is silicon coated polyester available, either in the form of the so-called "spintex," or tarps from Wilderness Experience and other companies. But either way, the expense is very high compared to nylon. Note that nylon has been an often used fabric for backpacking tents for decades, without sagging of the kind experienced by me and some of the posters on this thread. So perhaps it is the small denier, or very thin quality of what we call "silnylon" that contributes to this problem. Also, the arrival of single wall tents with minimal supporting structure aggravates the problem, making it much more likely that the sagging will get us wet.
Agree with Mat and David that when the weather cycles back and forth between moist and dry, tighteners and tensioners do not solve the problem in one operation. After all, a tent is primarily a place to sleep, not have to fumble with adjusting gear in the middle of the night. I have pitched a Tarptent in my yard, and watched it repeatedly alternate with the weather between taut and sagging for several days while I was repeatedly adjusting and tightening it. The swings between these extremes were so wide, that I knew it would perform poorly in strong winds when damp, and was afraid it would rip itself apart, or at least compromise its weatherproofness, if I continued to leave it up in dry sunshine. For that reason, I decided not to use the tent for serious trekking, where there is no hasty retreat. As said, I'm sticking with silnylon, but only what tests well, and only over a dome frame that minimizes the effect of sagging on comfort. Unfortunately, after much break testing, I do not expect carbon poles to withstand serious gales, and alloy poles are quite heavy, so that continues to be a problem. Roger Caffin's articles and many posts about his tents go a long way to addressing this issue, and it is not an insoluable problem. Just one that will require a good deal of thought and innovation with materials of consistently high quality. In the meantime, IMO, ample if not full interior netting, internal line tighteners and lots of well-supported headroom are a priority.
We live in a land fond of instant gratification, but this is one issue where some will have to put in a lot of sweat equity to get the kind of shelter many want. Unless you are one who is comfortable with just a tarp, in which case you have my sincere admiration; but I am one who, as the blues singer said, is 'built for comfort not for speed.'
See you in the mountains, Sam